LED (Light Emitting Diode) is a solid semiconductor device that direct transmits electric energy to visible light. It has advantages such as easy control, low voltage DC driving, and long life-time. Currently, LED is widely applied in many occasions, for example, lighting, display backlit, and display screen.
For preventing voltage spike damaging LED devices when LED driver system is starting, soft-start technology is conventionally applied on LED driver system. This technology comprises charging a start capacitor in controller, and so that voltage across LED device could rise gently until the start of LED driver is completed.
FIG. 1 illustrates a schematic circuit diagram of a soft-start LED driver controller 10 as a prior art. As shown in FIG. 1, when the system is starting, LED driver controller 10 receives a feedback signal VFB from LED string. An error amplifier 101 generates a compensating signal COMP according to the feedback signal VFB and a reference signal VREF. The compensating signal COMP is derived by the error amplifier 101 sourcing or sinking current to a compensating network 102 in which a compensating capacitor CCOMP and a compensating resistor RCOMP are coupled in series. As the voltage across the compensating capacitor CCOMP could not change suddenly, the compensating signal COMP rises gently during the system starting. The compensating signal COMP is compared with a sensed slope signal Slope indicative of a load current of LED string via a comparator 103. The comparator 103 generates a control signal CTRL to control a LED driver 104 to supply energy to LED strings.
However, after the soft-start is completed, the compensating capacitor CCOMP should be discharged to make the system restore to steady state. As the discharging process takes time, the compensating signal COMP may not fall to target value in time. Accordingly, a voltage spike may be generated on the voltage across the LED load after the starting is completed and before the system works in steady state. When the LED load is relatively light, the voltage spike may lead to voltage overshot, so as to trigger OVP (over-voltage protection) or OCP (over-current protection), or even causing damage.